feat: add metadata field to tag object

[chenjiahan]

Summary

This PR adda a new metadata field to the tag object:

See the added documentation to learn more.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR introduces a new metadata field on HTML tag objects so plugins can attach arbitrary data without affecting the generated HTML.

• Adds metadata to type definitions and serialization logic in the core
• Updates RsbuildHtmlPlugin to propagate metadata between internal and public tag formats
• Extends docs in both languages and adds an end-to-end test case for metadata handling

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
website/docs/zh/plugins/dev/hooks.mdx	Added metadata property to HtmlBasicTag type in Chinese docs
website/docs/en/plugins/dev/hooks.mdx	Added metadata property to HtmlBasicTag type in English docs
website/docs/zh/config/html/tags.mdx	Documented new metadata option in HTML tags config (Chinese)
website/docs/en/config/html/tags.mdx	Documented new metadata option in HTML tags config (English)
packages/core/src/types/config.ts	Added metadata to HtmlBasicTag TypeScript definition
packages/core/src/rspack/RsbuildHtmlPlugin.ts	Updated formatBasicTag and fromBasicTag to handle metadata
e2e/cases/plugin-api/plugin-modify-tags-meta/...	Added example plugin, config, and test to verify metadata usage
e2e/cases/plugin-api/plugin-modify-tags-meta/src/index.js	Placeholder entry file for the e2e test

Comments suppressed due to low confidence (1)

website/docs/en/plugins/dev/hooks.mdx:545

• [nitpick] It may be helpful to include a brief example or note here explaining how plugins can leverage the metadata field in the modifyHTMLTags hook.

  metadata?: Record<string, any>;